Low-density foams irradiated by a 20 kJ laser at the Omega laser facility (Laboratory for Laser Energetics, Rochester, NY, USA) are shown to convert more than 5% of the laser energy into 4.6 to 6.0 keV x rays. This record efficiency with foam targets is due to novel fabrication techniques based on atomic-layer-deposition of Ti atoms on an aerogel scaffold. A Ti concentration of 33 at. % was obtained in a foam with a total density of 5 mg/cm3. The dynamics of the ionization front through these foams were investigated at the 1 kJ laser of the Gekko XII facility (Institute for Laser Engineering, Osaka, Japan). Hydrodynamic simulations can reproduce the average electron temperature but fail to predict accurately the heat front velocity in the foam. This discrepancy is shown to be unrelated to the possible water adsorbed in the foam but could be attributed to effects of the foam micro-structure.

The strength of shock-loaded single crystal tantalum [100] has been experimentally determined using in situ broadband x-ray Laue diffraction to measure the strain state of the compressed crystal, and elastic constants calculated from first principles. The inferred strength reaches 35 GPa at a shock pressure of 181 GPa and is in excellent agreement with a multiscale strength model [N. R. Barton et al., J. Appl. Phys. 109, 073501 (2011)], which employs a hierarchy of simulation methods over a range of length scales to calculate strength from first principles.

Porosity generally embrittles ceramics. In contrast to such expectations, this report demonstrates that an effective fracture strain of nanoporous silica aerogels increases with increasing porosity. At ultralow relative densities of < 0.5%, nanoporous monoliths start exhibiting super-compressible deformation with effective fracture strains of > 50%. This is attributed to consequences of an increase in the aspect ratio of ligaments with decreasing density.

The performance of new iron-based laser-driven x-ray sources has been tested at the OMEGA laser facility for production of x rays in the 6.5–8.5 keV range. Two types of targets were experimentally investigated: low-density iron oxide aerogels (density 6−16 mg/cm3) and stainless steel foil-lined cavity targets (steel thickness 1−5 μm). The targets were irradiated by 40 beams of the OMEGA laser (500 J/beam, 1 ns pulse, wavelength 351 nm). All targets showed good coupling with the laser, with <5% of the incident laser light backscattered by the resulting plasma in all cases (typically <2.5%). The aerogel targets produced Te=2 to 3 keV, ne=0.12−0.2 critical density plasmas yielding a 40%–60% laser-to-x-ray total conversion efficiency (CE) (1.2%–3% in the Fe K-shell range). The foil cavity targets produced Te∼ 2 keV, ne∼ 0.15 critical density plasmas yielding a 60%–75% conversion efficiency (1.6%–2.2% in the Fe K-shell range). Time-resolved images illustrate that the volumetric heating of low-density aerogels allow them to emit a higher K-shell x-ray yield even though they contain fewer Fe atoms. However, their challenging fabrication process leads to a larger shot-to-shot variation than cavity targets.

Many applications of low-density nanoporous dielectrics are limited by their poor mechanical properties. Although nanoindentation is often used to evaluate the mechanical behavior of such materials, the physical meaning of the inelastic parameters measured by various indentation methods is not clear. Here, we study low-density nanoporous silica monoliths (aerogels) by nanoindentation using the most common indenter geometries (spherical, pyramidal and flat punches) and discuss the parameters measured. Results suggest that the deformation of the nanoporous silica monoliths studied in this work is controlled by elastic bending and fracture of nanoligaments with no signs of plasticity. The contact pressure (Meyer hardness) increases with increasing strain and does not represent the foam “crushing pressure”. The critical load for Hertzian fracture obeys the Auerbach law, and the formation of radial cracks and brittleness are strongly suppressed by the presence of nanopores. We also discuss the choice of indenter geometry and provide recommendations that can be used to overcome some typical challenges of indentation studies of low-density nanoporous solids, including very low contact stiffness for indenters with small contact areas, large surface roughness inherent to this class of materials, spatial non-uniformity (skin layers on monoliths), viscoelasticity and elastic nonlinearity.

The x-ray yields from laser-irradiated germanium-doped ultra-low-density aerogel plasmas have been measured in the energy range from sub-keV to ≈15 keV at the OMEGA laser facility at the Laboratory for Laser Energetics, University of Rochester. The targets’ x-ray yields have been studied for variation in target size, aerogel density, laser pulse length, and laser intensity. For targets that result in plasmas with electron densities in the range of ≈10% of the critical density for 3ω light, one can expect 10–11 J/sr of x rays with energies above 9 keV, and 600–800 J/sr for energies below 3.5 keV. In addition to the x-ray spectral yields, the x-ray temporal waveforms have been measured and it is observed that the emitted x rays generally follow the delivered laser power, with late-time enhancements of emitted x-ray power correlated with hydrodynamic compression of the hot plasma. Further, the laser energy reflected from the target by plasma instabilities is found to be 2%–7% of the incident energy for individual beam intensities ≈1014–1015 W/cm2. The propagation of the laser heating in the target volume has been characterized with two-dimensional imaging. Source-region heating is seen to be correlated with the temporal profile of the emitted x-ray power.

Experiments have been conducted using laser-driven cylindrical hohlraums whose walls are machined from Ta2O5 foams of 100 mg/cc and 4 g/cc densities. Measurements of the radiation temperature demonstrate that the lower density walls produce higher radiation temperatures than the high density walls. This is the first experimental demonstration of the prediction that this would occur [M. D. Rosen and J. H. Hammer, Phys. Rev. E 72, 056403 (2005)10.1103/PhysRevE.72.056403]. For high density walls, the radiation front propagates subsonically, and part of the absorbed energy is wasted by the flow kinetic energy. For the lower wall density, the front velocity is supersonic and can devote almost all of the absorbed energy to heating the wall.

Nanoporous structures are exceptionally useful in catalytic, sensing and mechanical applications. However, precise control over the structure and composition of the nanoporous material is critical for the material to behave as desired. We report here a new bottom-up synthesis technique termed filter-casting for the creation of large scale (>1 cm) nanoporous structures which provide this precise control. Cu, CuO and Cu2O and bi-modal macro/nanoporous Cu structures were created with this technique to demonstrate the range of materials and structures which can be formed into nanoporous monoliths. Homogeneous nanoporous monoliths are synthesized using nanoparticles, and bi-modal or higher-order porosities are achieved using a sacrificial polystyrene template. The higher-order pore size is determined by the polystyrene particle diameter, and the nanopore size is set by the diameter of the nanoparticles. Surface areas as high as 34 m(2) g(-1), and relative densities between 12 and 58%, have been achieved. Filter-casting is a powerful new method for directly synthesizing large nanoporous monoliths with predetermined composition, pore size and pore structure.

Homogeneous, nanocrystalline La1-xSrxMnO3 (LSM) powders, with x similar to 0.15, were synthesized with the epoxide addition sol-gel method. Through this simple technique, sol-gel materials were prepared from methanolic solutions of metal chlorides without the need for alkoxides, polymeric gel agents, or elaborate reaction schemes. The gels were dried in ambient conditions, resulting in mesoporous xerogels with networked nanostructures interconnecting particulate regions. Calcination of the dried gels resulted in the crystallization of single-phase LSM by 700 degrees C, after decomposition of intermediate hydroxide, chloride, and oxychloride compounds. SEM analysis indicated that the calcined powders were nanocrystalline and consisted of discrete particles, free of hard agglomeration. The average crystallite size and equivalent spherical diameter determined from XRD line broadening and BET analysis, respectively, were in good agreement with the SEM results (similar to 100 nm). The activation energies for the electronic conductivity from 1000 to 400 degrees C of sintered LSM xerogels were from 0.13 to 0.15 CV, in excellent agreement with previous reported results for LSM with x similar to 0.15.

Potential of aerogels for technological applications is often limited by their poor mechanical properties. Here, we demonstrate that alumina aerogel monoliths with excellent mechanical properties can be made by controlling the crystallographic phase, shape, and size of nanoligaments. In particular, we show that thermal processing of aerogels with a morphology of interconnected nanoleaflets causes dehydration and associated curling of the nanoleaflets, resulting in a dramatic improvement of mechanical properties. This study shows an effective way to control mechanical properties of the nanoporous solids that can be synthesized with ligaments having a quasi-two-dimensional shape, such as platelets, ribbons, or leaflets.

Supermolecular assembly techniques combined with the cation exchange of nanoparticles transforms mesostructured US into mesostructured CuS (see figure). The resulting materials retain the overall integrity of the original mesostructured material after the cation-exchange process.

We report a sol-gel synthesis of monolithic three-dimensional GeO2 nanostructures (aerogels). Transmission electron microscopy shows that these ultralow-density (∼99.2% porous) nanostructures are formed by a continuous network of elongated amorphous GeO2 nanoligaments, ∼15–20nm in diameter and ∼50–200nm in length. As a result, monoliths exhibit a high surface area of ∼60m2g−1. A comparison of soft x-ray absorption spectra at GeL3 and OK edges with theoretical densities of states reveals that the electronic structure of aerogels is similar to that of hexagonal GeO2, indicating a (distorted) tetrahedral environment of Ge atoms in the amorphous nanostructure.

Homogeneous, nanocrystalline powders of yttria-stabilized zirconia (YSZ) were prepared using a non-alkoxide sol-gel method. Monolithic gels, free of precipitation, were prepared by addition of propylene oxide to aqueous solutions of Zr4+ and Y3+ chlorides at room temperature. The gels were dried with supercritical CO2(l), resulting in amorphous aerogels that crystallized into stabilized ZrO2 following calcination at 500 degrees C. The aerogels and resulting crystalline products were characterized using in situ temperature profile X-ray diffraction, Raman spectroscopy, thermal analysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen adsorption/desorption analysis, and elemental analysis by inductively coupled plasma-atomic emission spectroscopy. TEM and N-2 adsorption/desorption analysis of an aerogel prepared by this method indicated a porous network structure with a high surface area (409 m(2)/g). The crystallized YSZ maintained high surface area (159 m2/g) upon formation of homogeneous, nanoparticles (similar to 10 nm). Ionic conductivity at 1000 degrees C of sintered YSZ (1500 degrees C, 3 h) was 0.13 +/- 0.02 Omega(-1) cm(-1). Activation energies for the conduction processes from 1000 to 550 degrees C and 550-400 degrees C were 0.95 +/- 0.09 and 1.12 +/- 0.05 eV, respectively.

Monolithic nanocrystalline anatase titania aerogels are synthesized by the epoxide sol-gel method followed by thermal annealing at 550 degrees C. These aerogels are formed by similar to 10-20 nm size anatase nanoparticles which are randomly oriented and interconnected into an open-cell solid network. Aerogel monoliths have an apparent density of similar to 10% and a surface area of similar to 100 m(2) g(-1). High-resolution transmission electron microscopy and soft X-ray absorption near-edge structure spectroscopy reveal good crystallinity of the anatase nanoparticles forming the aerogel skeleton. Published by Elsevier B.V.

A sol−gel approach for the synthesis of hybrid nanocomposites of iron oxide and bridged polysilsesquioxanes has been established. The procedures allow for the simultaneous formation of iron oxide and polysilsesquioxane networks in monolithic xerogels and aerogels. These hybrid nanocomposites are synthesized from FeCl3·6H2O and functionalized silsesquioxane monomers in a one-pot reaction using epoxides as a gelation agent. The porosity and microstructure of the materials have been determined by nitrogen porosimetry, electron microscopy, and ultra small-angle X-ray scattering. The hybrid nanocomposites exhibit a uniform dispersion of both components with no evidence for phase separation at length scales >5 nm. At this limit of resolution, it is not possible to distinguish between two independent interpenetrating networks integrated at molecular length scales or a random copolymer or mixtures of both.

Preparation of a low-density, high-surface-area SnO2 aerogel (see Figure inset), comprised of interconnected, randomly oriented crystalline (rutile) SnO2 nanoparticles similar to 3-5 nm in size is reported. X-ray absorption near-edge structure spectroscopy at the Sri M-4.5 edge reveals that the electronic structure of the SnO2 aerogel is similar to that of tetragonal SnO, rather than SnO2 or beta-Sn, with additional Sn-related electronic states close to the conduction band minimum.

We report on atomic layer deposition of an ∼2-nm-thick ZnO layer on the inner surface of ultralow-density (∼0.5% of the full density) nanoporous silica aerogel monoliths with an extremely large effective aspect ratio of ∼105 (defined as the ratio of the monolith thickness to the average pore size). The resultant monoliths are formed by amorphous-SiO2 core/wurtzite-ZnO shell nanoparticles which are randomly oriented and interconnected into an open-cell network with an apparent density of ∼3% and a surface area of ∼100m2g−1. Secondary ion mass spectrometry and high-resolution transmission electron microscopy imaging reveal excellent uniformity and crystallinity of ZnO coating. Oxygen K-edge and Zn L3-edge soft x-ray absorption near-edge structure spectroscopy shows broadened O p- as well as Zn s- and d-projected densities of states in the conduction band.

We describe a two-step dealloying/compaction process to produce nanocrystalline Au. First, nanocrystalline/nanoporous Au foam was synthesized by electrochemically driven dealloying. The resulting Au foams exhibited porosities of ∼60% with pore sizes of 40 and 100 nm and a typical grain size of <50 nm. Second, the nanoporous foams were fully compacted to produce nanocrystalline monolithic Au. The compacted Au was characterized by transmission electron microscopy and x-ray diffraction and tested by depth-sensing nanoindentation. The compacted nanocrystalline Au exhibited an average grain size of <50 nm and hardness values ranging from 1.4 to 2.0 GPa, which were up to 4.5 times higher than the hardness values obtained from polycrystalline Au.